The present invention relates to a process for the production of water-absorbing polymer structures. The invention also relates to the water-absorbing polymer structures obtainable by this process, water-absorbing polymer structures, a composite material, a process for the production of a composite material, the composite material obtainable by this process, chemical products, such as foams, shaped articles, fibers, foils, films, cables, sealing materials, liquid-absorbing hygiene articles, carriers for plant and fungal growth-regulating agents, packaging materials, soil additives or building materials, the use of water-absorbing polymer structures and the use of thermoplastic polymers.
Superabsorbers are water-insoluble, crosslinked polymers which are capable, by swelling and formation of hydrogels, of taking up large amounts of water, aqueous liquids, in particular body fluids, preferably urine or blood, and retaining these under pressure. Superabsorbers preferably absorb at least 100 times their own weight of water. Further details on superabsorbers are disclosed in “Modern Superabsorbent Polymer technology”, F. L. Buchholz, A. T. Graham, Wiley-VCH, 1998”. As a result of these characteristic properties, these water-absorbing polymers are chiefly incorporated into sanitary articles, such as, for example, babies' diapers, incontinence products or sanitary towels.
The superabsorbers currently commercially available are essentially crosslinked polyacrylic acids or crosslinked starch-acrylic acid graft polymers, in which some of the carboxyl groups are neutralized with sodium hydroxide solution or potassium hydroxide solution. These are obtainable by subjecting monomeric acrylic acid or salts thereof to free-radical polymerization in the presence of suitable crosslinking agents. In this context, various polymerization processes can be used, such as, for example, solution polymerization, emulsion polymerization or suspension polymerization. In the end, water-absorbing polymers in particulate form having a particle diameter in a range of from 150 to 850 μm are obtained by these various processes, and are then incorporated into the sanitary articles.
In order to improve the absorbency and swellability of these water-absorbing polymer particles, numerous processes have been described in which the surface of the polymer particles is modified. It is thus known, for example, from DE 40 20 780 C1 to react the water-absorbing polymer particles with alkylene carbonates, which can react with the carboxyl groups of the polymer particles. The surface post-crosslinking affected in this manner leads to an increase in the absorption of the polymers under the action of a pressure.
In addition to the reaction of the polymer composition with reactive compounds, numerous processes with which the properties of the water-absorbing polymer particles, in particular the permeability thereof, can be achieved by a coating with inorganic or organic fine particles are also described in the prior art.
Thus, DE 35 03 458 A1 describes that the absorption capacity, the rate of absorption and the gel strength of superabsorber particles can be improved by application of inert inorganic powder materials, such as, for example, silicon dioxide, in the presence of post-crosslinking agents.
To reduce the hygroscopicity and therefore to reduce the caking of the polymer particles, EP 0 388 120 A1 proposes coating the polymer particles with a porous powder of highly pure silicon dioxide, the powder having an average particles size of from 0.1 to 30 μm and a specific surface area of 500 m2/g.
However, all these processes for subsequent modification have the common feature that they lead to a release of fine particles in the form of dust, these fine particles being formed by mechanical stress, such as, for example, by pneumatic conveying and the resultant abrasion of the water-absorbing polymer particles. In addition to the associated dust pollution, a further disadvantage in the use of inorganic fine particles for surface modification of water-absorbing polymer structures lies in that fact that if these inorganic fine particles are employed during the surface crosslinking in particular, the current uptake of the post-crosslinking reaction can be comparatively high.
The present invention was based on the object of overcoming the disadvantages emerging from the prior art.
In particular, the present invention was based on the object of providing water-absorbing polymer structures which show comparatively low formation of dust and furthermore can be employed in hygiene articles, even in a high concentration, without the so-called “gel blocking” phenomenon occurring. The water-absorbing polymer structures should moreover be as easy as possible to meter in a device for the production of hygiene articles.
The present invention was also based on the object of providing a process for the production of water-absorbing polymer structures having the properties described above, which can also be carried out without the use of finely divided inorganic particles.